Original Article A Novel Ultrasound-Based Registration for Image- Guided Laparoscopic Liver Ablation Matteo Fusaglia MSc1, Pascale Tinguely, MD2, Vanessa Banz, MD2, Stefan Weber, PhD1, and Huanxiang Lu, PhD1 Abstract Background. Patient-to-image registration is a core process of image-guided surgery (IGS) systems. We present a novel registration approach for application in laparoscopic liver surgery, which reconstructs in real time an intraoperative volume of the underlying intrahepatic vessels through an ultrasound (US) sweep process. Methods. An existing IGS system for an open liver procedure was adapted, with suitable instrument tracking for laparoscopic equipment. Registration accuracy was evaluated on a realistic phantom by computing the target registration error (TRE) for 5 intrahepatic tumors. The registration work flow was evaluated by computing the time required for performing the registration. Additionally, a scheme for intraoperative accuracy assessment by visual overlay of the US image with preoperative image data was evaluated. Results. The proposed registration method achieved an average TRE of 7.2 mm in the left lobe and 9.7 mm in the right lobe. The average time required for performing the registration was 12 minutes. A positive correlation was found between the intraoperative accuracy assessment and the obtained TREs. Conclusions. The registration accuracy of the proposed method is adequate for laparoscopic intrahepatic tumor targeting. The presented approach is feasible and fast and may, therefore, not be disruptive to the current surgical work flow. Keywords image guidance, laparoscopic ablation, liver, registration Introduction needle’s trajectory is appropriate. Although laparoscopic-assisted ablation is more invasive than Surgical resection is considered the gold standard for the transcutaneous needle placement, it is less invasive than treatment of metastatic colorectal liver lesions, but 70% open-surgery liver ablations, thus providing the benefits to 90% of the patients are not eligible for this 1,2 related to laparoscopic surgery (ie, reduced scar size, procedure. This high rate has encouraged the use of trauma, and hospital stay). alternative surgical approaches such as microwave or During laparoscopic-assisted percutaneous needle- radiofrequency tumor ablation. For ablation, the based ablation, the surgeon has to align the tumor’s accurate placement of the needle’s tip in the proximity center and the needle within the US image. This of the tumor center has been shown to significantly 1 alignment requires great concentration and in-depth reduce the recurrence rate. experience,3 especially when tumors are poorly visible A laparoscopically assisted, percutaneous needle- in the US image. based ablation procedure, which allows tumor ablation This disadvantage has been reduced by use to be performed less invasively than open liver ablation employment of image-guided surgery (IGS) systems.4 procedures, has been proposed with the aid of 2D 3 Common IGS systems display the position of tracked ultrasound (US) scanning. Compared with surgical instruments relative to preoperative 3D virtual transcutaneous needle-based ablation, where a needle is models of the organ (reconstructed from CT or magnetic inserted percutaneously through the aid of computed tomography (CT) imaging, this approach provides additional visual guidance in the form of laparoscopic imaging. The use of laparoscopic imaging enables an 1University of Bern, Bern, Switzerland immediate control of the needle’s trajectory, from the 2University Hospital of Bern, Bern, Switzerland abdominal wall to the liver parenchyma, thus allowing a more intuitive needle insertion. Additionally, this Corresponding Author: approach eliminates the need of control scans—and, Matteo Fusaglia, ARTORG Center for Biomedical Engineering consequently, intraoperative radiation—which are Research, University of Bern, Murtenstrasse 50, 3010 Bern, required in interventional radiology to confirm that the Switzerland. Email: [email protected] Original Article Table 1. Summary of Registration Methods That Use Ultrasound Imaging. Work Registration Method Accuracy Application Applied Registration Bao et al6 Common landmarks between 3D Localization error: Laparoscopic Phantom reconstruction and the phantom 5.3 mm ablation/resection Krücker et al9 Common landmarks between Root-mean-square (RMS) Needle insertion Phantom preoperative 3D reconstruction and registration error: intraoperative US imaging 1.1 mm Martens et al15 Two stages: RMS error: 7.8-9.1 mm Ablation/resection Phantom/planned 1. Coarse alignment: based on (depending on animal trial landmark registration (acquired different magnetic with US probe) interferences) 2. Fine registration: surface scan resonance imaging). These are then mapped with the Despite promising results being reported, the main available intraoperative data (intraoperative US, disadvantage of these techniques is the time required to organ’s position, CT imaging). This mapping, also accurately define and detect the anatomical landmarks.16 called registration, determines a mathematical This aspect may derive from the reduced spatial relationship from the preoperative 3D model orientation and the lack of tactile feedback during coordinate system to the intraoperative image laparoscopic procedures. We hypothesize that a coordinate system.5,6 Obtaining an accurate promising approach to achieving an accurate and registration represents a key aspect of the successful efficient registration is to use 3D reconstructions of the clinical use of IGS technologies in surgery. hepatic vasculature from LUS. By performing a LUS Several research groups have reported the use of sweep over the intrahepatic region of interest (ROI; ie, registration techniques that exploit the detection of in the proximity of tumors), an intraoperative 3D superficial anatomical features of the liver (eg, skin volume of the underlying vessels can be reconstructed. fiducials, anatomical landmarks) through tracked Subsequently, this intraoperative US-based 3D volume instruments.7-14 Although these methods provide can be accurately registered to the preoperative 3D accurate registration on the liver surface, they lack model. The registration accuracy, defined as target accuracy at the intraparenchymal structures (eg, registration error (TRE), is expected to be <10 mm, tumors), thus hindering precise targeting of which is commonly suggested as a safety margin.17 intrahepatic lesions. Additionally, we believe that because sweeping a LUS Superior accuracy may be provided by is more similar to intraoperative actions than defining intraoperative US because it visualizes intrahepatic landmarks, this technique would lead to a fast and structures that are closer to clinical targets.8 intuitive work flow. Whereas previous works15,18,19 have Registration techniques that exploit the detection of reported the use of US-based 3D volumes, in the context intraparenchymal hepatic structures (eg, vessel of enhanced US guidance, to date, this technique has not bifurcations, tumors) through US imaging have been been reported in the context of registration for reported in some studies.6,9,15 Through a calibrated laparoscopic IGS. and tracked laparoscopic ultrasound (LUS), In this work, we present a novel registration method that anatomical landmarks that lie in the US image planes reconstructs an intraoperative US-based 3D volume of are manually identified and matched with the the intrahepatic vessel anatomy and register it with the preoperative 3D model. A similar approach can also preoperative 3D model. To further ease the work flow, be found in 2 commercially available US systems we propose a scheme for intraoperative accuracy (ACUSON S3000, HELX, Siemens Healthcare assessment by visually inspecting the overlay of the GmbH, Germany), where, after the identification of preoperative 3D model on the intraoperative US. Our suitable landmarks, preoperative 3D reconstructions aim is to assess the registration accuracy by computing can be fused with the available US image. Although the TRE on a phantom with 5 intrahepatic target tumors, these systems provide advanced imaging visualization thus reflecting a clinically relevant scenario. modules, they are designed for obstetrics and Furthermore, the work flow efficiency is evaluated by pediatrics applications and not for laparoscopic analyzing the time required for performing the procedures. Table 1 provides a summary of the registration procedure and the relationship between the aforementioned registration methods. proposed intraoperative evaluation method and the obtained TREs. Fusaglia et al 3 Figure 1. System overview and functional components. Abbreviations: US, ultrasound; EM, electromagnetic; IGS, image-guided surgery. Materials and Methods 1. Preoperative planning: The preoperative 3D model is displayed on a touch monitor, allowing System Overview the surgeon to selectively visualize structural and functional analysis (eg, portal vein territories, A commercial IGS system for open liver procedures tumor volumetry). (CAScination AG, Switzerland) was supplemented with 2. Intraoperative imaging: The intraoperative US electromagnetic (EM) tracking (Aurora, Northern image is displayed on a second touch monitor Digital, Canada) and an interface to a LUS (Figure 1; and fed from an external US system (BK Flex800 with Probe 8666-RF, BK Medical, Denmark). Medical, Denmark). An interactive interface The EM field generator creates a magnetic field that